[clang] Introduce `SemaObjC` (#89086)

This is continuation of efforts to split `Sema` up, following the
example of OpenMP, OpenACC, etc. Context can be found in
https://github.com/llvm/llvm-project/pull/82217 and
https://github.com/llvm/llvm-project/pull/84184.

I split formatting changes into a separate commit to help reviewing the
actual changes.